Fig. 1. Diagram showing the nadir scanning mode of EMI-Ⅱ

Fig. 2. Spectral fitting results for some orbit on January 15, 2022 from EMI-Ⅱ. (a) Measured and reference spectra; (b) SO₂ differential optical thickness; (c) ring differential optical thickness; (d) remaining residual of DOAS fitting

Fig. 3. Diagram of deducting the stripe effect of some orbit. (a) Before deducting the stripe effect; (b) after deducting the stripe effect

Fig. 4. Monitored SO₂ results from the La Palma volcano. (a) Retrieved SO₂ VCD from EMI-Ⅱ on October 27, 2021; (b) SO₂ VCD obtained by TROPOMI on October 27, 2021; (c) retrieved SO₂ VCD from EMI-Ⅱ on October 29, 2021; (d) SO₂ VCD obtained by TROPOMI on October 29, 2021; (e) retrieved SO₂ VCD from EMI-Ⅱ on October 31, 2021; (f) SO₂ VCD obtained by TROPOMI on October 31, 2021

Fig. 5. Linear fitting results of SO₂ VCD between EMI-Ⅱ and TROPOMI at La Palma volcano area at different time. (a) October 27, 2021; (b) October 29, 2021; (c) October 31, 2021

Fig. 6. Monitored SO₂ VCD of Tonga submarine volcano. (a) Result from EMI-Ⅱ on January 14, 2022; (b) result from TROPOMI on January 14, 2022; (c) result from EMI-Ⅱ on January 15, 2022; (d) result from TROPOMI on January 15, 2022; (e) result from EMI-Ⅱ on January 16, 2022; (f) result from TROPOMI on January 16, 2022; (g) result from EMI-Ⅱ on January 17, 2022; (h) result from TROPOMI on January 17, 2022; (i) result from EMI-Ⅱ on January 18, 2022; (j) result from TROPOMI on January 18, 2022

Fig. 7. Diagram of the forward trajectories of air masses during the Tonga submarine volcano eruptions

Fig. 8. Scatter plots of SO₂ VCDs from EMI-Ⅱ and TROPOMI during Tonga submarine volcanic eruption. (a) January 15, 2022; (b) January 16, 2022; (c) January 17, 2022; (d) January 18, 2022

Table 1. Parameter settings of DOAS fitting for retrieving the SO₂

Table 2. Parameter nodes for building the SO₂ AMF look-up table